This invention relates to coupling between narrow- and wide-channel optical waveguides and especially to parabolic horns for coupling between such waveguides.
Many devices in integrated optics require transitions between planar waveguides or wide-channel waveguides, which support many modes, and narrow-channel waveguides, which support only a single mode. For these horn-shaped transition regions to efficiently transfer optical power, the first-order mode should propagate adiabatically through the device without mode conversion to higher-order modes or to radiation modes. The problem of the design of tapered horn-shaped structures for minimum length has been considered theoretically by Winn and Harris in Applied Optics 14, 3012 (1975) and by Nelson in IEEE Trans. Microwave Memory Tech. MTT-23, 92 (1975). They give coupling efficiencies for various length linear tapers in glass waveguide systems and Winn and Harris showed that a given coupling efficiency could be achieved in a shorter length by using an exponential taper. Unfortunately these results are not conveniently adaptable to other waveguide systems, such as the metal-ion diffused ferro-electrics. To date no experimental coupling efficiencies have been published.
The design given herein is for a shaped coupling horn based on considerations of adiabatic propagation in waveguides and a desire to minimize length. In the limit of well-confined modes far from cutoff, the criterion reduces to a design based on diffraction theory and the horn shape becomes parabolic. The design criteria can be easily applied to any waveguide system whose dispersion characteristics are known and to any wavelength, and is therefore completely general. Reported herein are some measurements of coupling efficiencies made with various-sized channel waveguides in Ti-diffused LiNbO.sub.3.